An AC generator having an armature coil and a field coil is generally known as one type of conventional rotating electrical machine for a vehicle.
The AC generator includes a rectifier having a rectifying element for rectifying an AC voltage formed by the armature coil, and a cooling fin to which the rectifying element is fixed.
The cooling air is generated by a cooling fan, which is fixed to a rotor of the AC generator and rotates integrally therewith, and while being introduced into a generator housing through inlets disposed in a rear end wall of the generator housing, the cooling air is brought into contact with the rectifying element and the cooling fin.
Then, the cooling air is discharged to an outside from outlets disposed in a peripheral wall of the generator housing after cooling an interior of the generator housing.
With respect to the cooling fin, Japanese Patent No. 4180385 discloses a technology in which a large number of cooling holes penetrating in an axial direction are provided in the cooling fin to form cooling air flow passages in the axial direction so that the cooling performance can be improved.
Since these cooling holes are normally formed by casting, a draft angle of a mold for forming a cooling hole is required, so that a cooling hole is formed into a tapered shape.
In recent years, the above-mentioned rotating electrical machine for the vehicle is desired to be small in size and high in output, and in order to cope with an increase in heat generation accompanying an improvement in output, improvement in cooling performance is required.
As a method for improving the cooling performance, although it is conceivable to improve capacities of the cooling fan and the cooling fin, improving the capacity of the cooling fan is undesirable as it involves an increase in noise.
Therefore, improvement of the cooling capacity of the cooling fin is required.
In such a case, as a method of improving the cooling capacity of the cooling fin with a limited fin space, the distance L between two adjacent cooling holes 156, 156 is shortened, as shown in FIG. 13, so that the number of the cooling holes 156 can be increased and the cooling holes 156 can be disposed densely.
However, a minimum wall thickness T between the two adjacent cooling holes 156, 156 is required to be equal to or more than a certain value in order to obtain the proper running property during casting and the strength of the cooling fin.
Further, on a surface of a cooling fin 153 opposite to a tapering direction (a bottom side in FIG. 13), a wall thickness T2 between the adjacent cooling holes 156, 156 increases more than necessary.
Therefore, it has been difficult to improve the cooling capacity by shortening the distance L between the cooling holes 156, 156 and by increasing the number of the cooling holes 156.
Further, since there is a difference in thickness between the cooling holes 156, 156 depending on positions in the thickness direction in the conventional cooling fin 153, there is a difference in strength.
In addition, since the rotating electrical machine is usually mounted at a position close to an engine of the vehicle, the cooling fin 153 receives a large exciting force due to engine vibration.
For this reason, the concentration of stress due to the difference in strength in the thickness direction of the cooling fin 153 cannot be ignored.
Therefore, there is a problem that a minimum wall thickness T must be taken large in order to obtain necessary strength.